1. Field of the Invention
This invention relates generally to a weighted surface acoustic wave (SAW) reflector for use in a SAW reflector filter or SAW resonator and, more particularly, to a weighted SAW reflector for use in a SAW reflector filter or resonator, where the reflector includes reflector grid lines selectively dithered relative to uniformly spaced grid lines that have M grid lines per each Nλ, where λ is the wavelength of the center frequency of the frequency band of interest, M and N are integers and M>N.
2. Discussion of the Related Art
Intermediate frequency (IF) filters are employed for channel selection in mobile phone communications systems, such as CDMA and GSM. The IF filters must be small in size and provide narrow bandwidths with steep transition edges and good out of band rejection. One type of filter that provides these properties is known in the art as a surface acoustic wave (SAW) filter.
Conventional SAW filters include an input transducer and an output transducer formed on a piezoelectric substrate. The input transducer is electrically excited with the electrical input signal that is to be filtered. The input transducer converts the electrical input signal to surface acoustic waves, such as Rayleigh waves, lamb waves, etc., that propagate along the substrate to the output transducer. The output transducer converts the acoustic waves to a filtered electrical signal.
The input and output transducers typically include interdigital electrodes formed on the top surface of the substrate. The shape and spacing of the electrodes determines the center frequency and the band shape of the acoustic waves produced by the input transducer. Generally, the smaller the width of the electrodes, or the number of electrodes per wavelength, the higher the operating frequency. The amplitude of the surface acoustic waves at a particular frequency is determined by the constructive interference of the acoustic waves generated by the transducers.
The combined length of the transducers determines the length of the overall filter. To design a conventional SAW filter with ideal filter characteristics, the filter's impulse response needs to be very long. Because the length of the impulse response is directly proportional to the length of the transducer, the overall length of a conventional SAW filter having ideal characteristics would be too long to be useful in mobile phone communications systems.
Reflective SAW filters have been developed to satisfy this problem. Reflective SAW filters generally have at least one input transducer, one output transducer and one reflector formed on a piezoelectric substrate. The reflector is typically a reflective grating including spaced apart grid lines defining gaps therebetween. The acoustic waves received by the reflector from the input transducer are reflected by the grid lines within the grating so that the reflected waves constructively and destructively interfere with each other and the wave path is folded. The constructively interfered waves are reflected back to the output transducer having a particular phase. Because of the folding, the length of the transducer is no longer dependent on the duration of the impulse response. Reflective SAW filters are, therefore, smaller in size and have high frequency selectivity, and thus are desirable for mobile phone communication systems.
The frequency response of a reflective SAW filter is further improved by weighting the individual reflectors to achieve a desired net reflectivity. Existing weighting methods include position-weighting, omission-weighting, and strip-width weighting. Other methods of weighting reflectors include changing the lengths of open-circuited reflective strips within an open-short reflector structure. Weighting the reflector helps to reduce the physical size of the filter and to improve the filter's frequency response.
An ideal frequency response for a reflector SAW filter has steep transition edges. The reflective gratings in a reflector SAW filter are weighted by a suitable weighting function to provide the desired filter response. For example, a weighted sin(x)/x function can be implemented in each reflective grating to generate a filter response having very steep transition edges.
Existing weighting techniques include position-weighting, omission-weighting and strip-width weighting. Other methods of weighting reflective gratings include changing the length of open-circuited reflective strips within an open-short reflector structure. Weighting the reflective grating helps to reduce the physical size of the filter and improve the filters frequency response.
The weighted reflective grating acts as a key element in the reflector SAW filter by reducing the physical size of the filter and improving the electrical filter response. A size reduction of 70% and an insertion loss of around 8 dB has been reported in the art using a Z-path reflector filter compared to in-line filter structures. One known reflective filter is a Z-path IF SAW filter for CDMA mobile phones.
The known methods of weighting a reflective grating in a SAW filter are all dependent upon the critical dimension of the reflector structure. The critical dimension is the smaller of the reflective grating grid width or the gap width, and is inversely proportional to the operating frequency of the filter. As the operating frequency increases, the critical dimension decreases. Fabrication constraints limit the critical dimension, thus limiting the operating frequency of the filter. As the operating frequency of the filter increases, the known reflective gratings have a limited dynamic range when implementing a wide range of reflectivity, which is required for filters with high selectivity. A reflective grating that provides strong reflectivity at a given frequency and critical dimension would be advantageous.